The proposed studies will examine the mechanism whereby overexpression of a NF-L transgene leads to motoneuron degeneration in transgenic mice. The impetus for the studies derives from recent discoveries that the level of NF-L expression is regulated by altering the stability of NF-L mRNA and that mRNA stability is, in turn, determined by RNA binding complexes that bind to the 3'UTR of the NF-L mRNA. If similar complexes also regulated the expression of gene products involved in neuronal homeostasis, titration of the complexes (or components thereof) by expression of a NF-L transgene may alter neuronal homeostasis and could lead to motoneuron degeneration. This working hypothesis is strongly supported by preliminary studies and the studies are highly relevant to motoneuron diseases, such as ALS. In order to test the working hypothesis, NF-L transgenes will be constructed so as to alter the composition and bindings sites of RNA- binding complexes and to compare the abilities of the different transgenes to induce motoneuron degeneration in transgenic mice. Preliminary studies have identified a 68-nucleotide binding site for an RNA-binding complex (C-binding complex) at the junction of the coding region and 3'UTR of the NF-L mRNA as the prime suspect for titrating factors that are responsible for causing motoneuron degeneration. The titration effects of the C- binding complex will be tested by deleting, multimerizing, and mutating the site and by repositioning the site in the NF-L mRNA or in a heterologous transcript. The adverse effect of transgene expression on motoneuron viability will also be examined using a tetracycline-inducible promoter whereby expression of the transgene can be regulated by addition or withdrawal of tetracycline to transgenic mice. The tetracycline- regulated system will allow a systematic and precise study of the developmental period of motoneuron vulnerability as well as the reversibility of the pathological phenotype. Finally, studies will also examine the possibility that alterations in post-transcriptional regulation may also occur in motoneuron degeneration from expression of a mutant SOD-1 transgene.